Array antennas utilizing grouped radiating elements

ABSTRACT

AN ARRAY ANTENNA WHEREIN THE RADIATING ELEMENTS ARE QUANTIZED INTO GROUPS OF ELEMENTS. EACH GROUP CONSISTS OF SIXTEEN ELEMENTS IN FIVE ADJACENT COLUMNS OF A TRIANGULAR GRID, HAVING FOUR ELEMENTS IN EACH OF THE THREE INNER COLUMNS AND TWO ELEMENTS IN EACH OF THE OUTER COLUMNS. IN EACH GROUP THE ELEMENTS ARE ARRANGED TO BE SYMMETRICAL ABOUT THREE AXES HAVING A COMMON INTERSECTION AND SEPARATED BY SIXTY DEGREES. THE ELEMENTS THAT COMPRISE EACH GROUP ARE COMMONLY EXCITED AND EACH GROUP OF ELEMENTS IS EXCITED BY A DIFFEERNT BASIC EXCITATION SIGNAL. THIS ARRANGEMENT SUBSTANTIALLY REDUCES THE NUMBER OF BASIC EXCITATION SIGNALS REQUIRED AND PROVIDES EFFICIENT EXCITATION OF EACH ELEMENT WITH LOW QUANTIZATION SIDELOBES.

Jan. 5, 19 1' a; c. CHARLTON 3,553,703

ARRAY ANTENNAS UTILIZING-GROUPED RADIATING ELEMENTS 4 Sheets-Sheet 1Filed July 25, 1968 FIG. la

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ARRAY ANTENNAS UTILIZING GROUPED RADIATING ELEMENTS Filed July 25, 19684Sheets-Sheet 2 GRATING LOBE MAIN LOBEI- aomw awn'obw BOiO-Vd AVUHV 4nmun-1.4mm

FIG 3b Jan; 5, 1971 G G C HARLTQ N i 3,553,706

I ARRAY ANTEflNAS UTILIZING GROUPED RADIATING ELEMENTS Filed July 25,1968 4 Sheets-Sheet s ARRAY FACTOR m MODULE FACTOR O E j g 2 t Z .J a. g

SIN 0 EDGE OF REAL SPACE". FOR MAXIMUM SCAN ANGLE i -QUANT'I/ZATIONSIDELOBES ALL TOTAL ANTENNA PATTERN AMPLITUDE SlN 9 FIG. 6-

Jan. 5, 1 cs. CHAR-LTQN 4 Sheets-Sheet 4.

Filed July 25; 1968 o o o I o o o o o o o o o o o o o b o o o o o o o oo o o o o o o o o o o o o o o o o o o o o o o o o o o o v n: o o o o o oo 0 o 0 o o o o o o o o o o o o 37 ELEMENTS 37 ELEMENTS 25 ELEMENTS o oo o f o o o o o o o 0 l9 ELEMENTS l6 ELEMENTS l6 ELEMENTS I3 ELEMENTS 8ELEMENTS 7 ELEMENTS l2 ELEMENTS 9 ELEMENTS o 0 o 0 00000 0 o o o o o P o.0 o o o D O O O O O 0 O O 0 O O O O O 36 ELEMENTS l2 ELEMENTS I8ELEMENTS FIG. 5

United States Patent US. Cl. 343--777 3 Claims ABSTRACT OF THEDISCLOSURE An array antenna wherein the radiating elements are quantizedinto groups of elements. Each group consists of sixteen elements in fiveadjacent columns of a tniangular grid, having four elements in each ofthe three inner columns and two elements in each of the outer columns.In each group the elements are arranged to be symmetrical about threeaxes having a common intersection and separated by sixty degrees. Theelements that comprise each group are commonly excited and each group ofelements is excited by a ditfeernt basic excitation signal. Thisarrangement substantially reduces the number of basic excitation signalsrequired and provides efiicient excitation of each element with lowquantization sidelobes.

This invention is directed to array antennas and more particularly tolarge arrays having a substantial number of individual radiatingelements arranged in more than one dimension. The invention isapplicable to planar arrays and may also find application in sphericalor other curved arrays.

For an array antenna, independently controlling the excitation amplitudeof each element provides optimum performance. However, for antennashaving a large number of radiating elements, independent excitation ofeach element requires a very complex and expensive signal generator andfeeding network. The signal generator and feeding network can besimplified by quantizing the radiating elements into identical groups ofmodules and feeding each group independently. However, such grouping ofelements can result in undesirable amplitude quantization sidelobes.Quantization sidelobes are analogous to grating lobes and result fromthe large spacing between the centers of each group of elementsindependently excited.

Objects of the present invention therefore are to provide new andimproved array antennas which provide efficient, economical excitationof the radiating elements and produce minimal amplitude quantizationsidelobes.

In accordance with the present invention there is provided in an arrayantenna, wherein grouping of the array elements reduces the number ofbasic excitation signals required to excite all the elements, anarrangement achieving efiicient excitation of the array elements withlow quantization sidelobes which comprises a plurality of identicalgroups of radiating elements, each group consisting of sixteen radiatingelements in five adjacent col umns of a triangular grid having fourelements in each of the three inner columns and two elements in each ofthe outer columns and arranged to be symmetrical about three axes havinga common intersection and separated by sixty degrees. The invention alsoincludes means for coupling a different basic excitation signal to eachgroup of elements, the signal coupled to each group being coupled to allof the elements in the group, whereby the prescribed grouping ofelements substantially reduces the number of basic excitation signalsrequired, provides efficient excitation of each element andsubstantially reduces the quantization sidelobes.

For a better understanding of the present invention together with otherand further objects thereof, reference "ice is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawings:

FIGS. 1a, 1b and 1c illustrate an array antenna system constructed inaccordance with the present invention;

FIGS. 2a and 2b illustrate a typical array factor with the apertureexcited in the azimuth difierence mode;

FIGS. 3a and 3b illustrate a typical module factor;

FIGS. 4a and 4b illustrate a total antenna radiation pattern of anantenna consisting of a plurality of groups of radiating elements;

FIG. 5 illustrates a plurality of groups of elements that can becombined with identical groupings to form an array; and

FIG. 6 is a graphical representation of the quantization sideloberseponse of the FIG. 5 groupings.

FIG. la illustrates a group 10 of sixteen radiating elements 12 arrangedin accordance with the teaching of the present invention. The elements12 are illustrated, by way of example, as circular openings 12 in ametal ground plane 26. Each of these openings 12 terminates a circularwaveguide and each propagates electromagnetic energy coupled thereto bythe corresponding circular waveguide. Any other suitable radiatingelement such as dipoles or slotted waveguides could also be utilized.

The group of sixteen elements 12 are arranged in five adjacent columns13, 14, 15, 16 and 17 of a triangular grid of elements, having fourelements in each of the three inner columns 14, 15, and I S, and twoelements in each of the outer columns 13 and 17. The elements 12 arearranged to be symmetrical about three axes 15, 18, and 19 having acommon intersection at 0 and separated by sixty degrees.

The technique of locating radiating elements in a triangular grid iswell known to those skilled in the art and need not be described indetail. Briefly stated, the elements are so arranged that each element.is located in a corner of an equilateral triangle of elements, asillustrated by triangle A.

FIG. 11; illustrates a portion 27 of an antenna consisting of aplurality of identical groups 10 of radiating elements, each group beingidentical to the groups illustrated in FIG. 1a. The groups of elements10 are joined together to form a large planar array, the portion 27 ofwhich is illustrated in FIG. 1b. The individual elements 12 thatcomprise each group are shown in the center 7 groups of elements inorder to illustrate that the grouping of FIG. 1a can be combined withidentical groups to form a continuous array in a triangular grid with noinactive elements. It is essential that the combining of identicalgroups such as illustrated in FIG. 1a provides a continuous array sincean inactive element (a position in the triangular grid Where there is noactive radiating element) produces deleterious effects in the arraypattern as indicated in conjunction with the discussion of FIG. 6.

FIG. 10 taken in conjunction with FIGS. 1a and 1b illustrates an antennasystem constructed in accordance with the present invention. Aspreviously stated, the system consists of a plurality of identicalgroups of radiating elements 10 with each group consisting of sixteenelements arranged as illustrated in FIGS. 1a and 111. A front view of aportion of the antenna is illustrated at 27 in FIG. 1b. Element 25 is aside view of the entire group of radiating elements including segment27.

The system further includes means, illustrated as connections a-n, forcoupling a different basic excitation signal from the signal generator111 to each group of elements, the signal coupled to each group 10 beingcoupled to all the elements 12 in said group. The signal generator 11also includes distribution network required to provide the desiredsignals to connections a-n.

For some applications, grouping of elements as described above isdesired only as an intercoupling arrange-ment; i.e., to permit couplinga different basic excitation signal to each group for reducing the basicnumber of excitation signals required. However, for other applicationsit may be desirable to manufacture each group as a separate module andsubsequently assemble the modules into a complete array. This is madepossible in the present invention by having each group of elementsconsist of adjacent elements, rather than having the elements of thedifferent groups interleaved.

In operation, a different one of the basic excitation signals is coupledfrom the signal generator 11 to the corresponding one of the groups ofelements that comprise the antenna 25, by way of the leads a-n. Each ofthe basic excitation signals is coupled to the sixteen radiatingelements 12 that comprise the corresponding group. For example, in FIG.10 the basic excitation signal coupled by the lead i to the antenna iscoupled to each element of one of the groups 10 of elements illustratedin FIG. 1b by the fifteen T junctions 20. As illustrated the T junctionsare arranged so that the basic excitation signal is divided into sixteensubstantially equal parts providing equal amplitude excitation of allthe elements that comprise that group. Each of the sixteen leads 28connected to the antenna 25 represents a portion of the waveguide whichis terminated by one of the circular openings 12 illustrated in FIG. 1b.

Similarly, each of the remaining basic excitation signals is coupled toone of the groups 10 of radiating elements by another group of fifteen Tjunctions so that there is equal amplitude excitation of all theelements that comprise the corresponding group. The nature of the basicexcitation signals coupled from signal generator 11 depends on the typesystem in which the invention is em ployed. The invention is equallyapplicable to any array; i.e., a phased array, a monopulse array, etc.

The arrangement of FIG. 1 provides sufficient improvements over theprior art. As illustrated in FIG. lc, quantizing the radiating elements12 into groups of sixteen, substantially reduces the number of basicexcitation signals supplied by means 11. Furthermore, since sixteen is abinary number, the FIG. 1 arrangement makes possible the efficient equalamplitude excitation of each element that comprises a group 10 byutilizing simple inexpensive T junctions 20 to couple the basicexcitation signal from means 11 to all of the elements that comprise thegroup. If the number of elements in each group were other than a binarynumber specially designed couplers would be required to achieve equalamplitude excitation of the elements that comprise each group.

Furthermore, the grouping of elements illustrated in FIGS. 1a and 1bprovides superior sidelobe performance. To illustrate this superiorperformance the following discussion describes the present inventionembodied in a monopulse system excited in the azimuth difference modeand compares the performance with other possible grouping arrangementssimilarly excited. This is done only to illustrate the comparativeperformance of the present invention and this invention is in no waylimited to monopulse systems.

In order to compare quantization sidelobes, it is necessary to obtainthe total antenna radiation pattern. The total antenna radiation patternof an array of elements which are grouped together in identical modulessuch as illustrated in FIG. 1b is determined by obtaining the product ofthe array factor the module factor and the element pattern. The arrayfactor is the pattern of isotropic radiators located at the center ofeach module 10. The module factor is the pattern of one module 10 withisotropic radiators located at each element 12. The element pattern,which is the pattern of one element 12 in the presence of the others,can be neglected since it is slowly varying with angle.

The array factor has many grating lobes in real space since the modulespacing is many element spacings E as shown in FIG. 1b. A typical arrayfactor with the aperture excited in the azimuth-difference mode is shownin FIGS. 2a and 2b where a equals the module spacing in the azimuthplane, b equals the module spacing in the elevation plane, 0 equals theangle from the main lobe and equals the free space wavelength. Both thegrating lobes and the main lobe shown have approximately the shape ofthe main lobe for the total antenna pattern. The spacing of the gratinglobe centers in a given direction depends upon the module spacing inthat direction, as indicated.

A typical module factor is shown in FIGS. 3a and 3b. The contoursplotted are the positions where the module factor nulls occur. Thesidelobe peaks occur between these nulls.

The total antenna pattern illustrated in FIGS. 4a and 4b is the productof the array factor and the module factor. FIG. 4a illustrates both thearray factor and the module factor in the azimuth plane from FIGS. 2 and3. FIG. 4b shows their product. For the diiflference mode as shown, thepeaks of the quantization sidelobes occur at angles near the peaks ofthe array-factor grating lobe. The antenna pattern grating lobe is alsoshown. The element spacing is chosen to keep this grating lobe fromentering real space for the maximum scan angle required.

FIG. 4a also indicates that the nulls of the module factor pass throughthe centers of the array-factor grating lobes. This can be proven bytaking the array and adding many more modules to extend it to aninfinite array, and then uniformly exciting it. The module factorremains unchanged, and the array factor has a zero-beamwidth main beamand grating lobes located at the same angles. Since the infiniteaperture must have only a single zerobeamwidth beam and no sidelobes,the array-factor grating lobes occur at the nulls of the module factor.For the finite array, the nulls of the module factor still occur at thecenters of the array-factor grating lobes; however, since thearray-factor beamwidth is not zero, the quantization sidelobes are thusformed.

Since the quantization sidelobes occur in the region of the array-factorgrating lobes, their level can be obtained by calculating only a smallportion of the pattern. It should also be noted that th difference-modequantization sidelobe levels are higher than that of the sum mode. Thisis because the difference-mode peaks occur off the array-factor gratinglobe centers and therefore result in a greater product of the twofactors than in the sum-mode. The sum-mode quantization sidelobe levelsare about 5 db lower than the difference-mode.

The quantization sidelobe levels are not greatly dependent on theaperture illumination in the difference mode. This is because the peakof the quantization sidelobes occur about at the peak of thearray-factor grating lobes of the difference mode and because thepositions of difference-mode peaks are not greatly dependent on theaperture illumination. The sum-mode quantization sidelobe levels arealso not greatly dependent on the aperture illumination, but do havemore variation that those of the difference mode.

FIG. 5 illustrates many of the various modules or groups of elementsthat have been evalulated, includinggrouping S-(f), the subject of thepresent invention. Each of these groupings can be combined withidentical groupings to form an array of a desired size. Many othergroupings were given primary consideration but discarded because theycould not be combined with identical groupings or they obviously hadhigh quantization sidelobes. As in group 5(f) each of these modulesconsists of elements in a triangular grid of elements and all of theelements in each group are active elements.

Although the modules must be large enough to provide simplicity ofexcitation the modules with the fewer number of elements obviously givelower quantization sidelobes. The smaller modules have broader modulefactors and therefore have their nulls farther off broadside. This givesa lower slope in the null region and therefore a lower quantizationsidelobe. However, it does not provide as much feeding simplicity.

FIG. 6 illustrates the highest difference mode sidelobes resulting fromthe quantization of the aperture excitation into the groups illustratedin FIG. 5. As expected, the modules with fewer elements give lowerquantization sidelobes in general. However, the grouping of 16 elementsillustrated in FIG. 1b and FIG. provides unexpectedly good results forits size, it being kept in mind that the larger the grouping, thesimpler the feeding network. The 5 (f) grouping of elements has betterquantization sidelobe response than the grouping of eight elementsillustrated in 5(j), the grouping of seven elements illustrated in 5(i), and the grouping of twelve elements illustrated in 5 (h). The onlyother grouping of sixteen elements that can be combined with identicalgroupings, grouping 5(e), has substantially poorer quantizationsidelobes. The only grouping that provides comparable quantizationsidelobes as the grouping of sixteen elements illustrated in 5 (f) isthe grouping of thirteen elements illustrated in 5 (g). However,thirteen is not a power of two and the feeding network for an arrayconsisting of identical 5 (g) grouping would be considerably morecomplicated than that required for an array consisting of 5(])groupings. Although the 5(g) grouping has good quantization sidelobeperformance it does not provide the simplicity of excitation provided bythe present invention. Similarly, the grouping of seven elementsillustrated at 5 (k) requires a complicated feeding network, wouldrequire more than double the basic excitation signals than the 5(grouping and the improvement in quantization sidelobe performance is notappreciable, considering the fact that the 5(k) grouping has less thanhalf of the elements of the 5(f) groupmg.

The points plotted at m, n, and p of FIG. 6 illustrate the consequencesof having an inactive element in any of the groupings. Grouping 5 (m), 5(n), and 5 (p) are identical to 5 (g), 5(d), and 5(a), respectively,except in the m, n, and p groupings the center element is inactive. Theeffect on the quantization sidelobe performance is evident from FIG. 6.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is, therefore, aimedto cover all such changes and modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. In an array antenna, wherein grouping of the array elements reducesthe number of basic excitation signals required to excite all theelements, an arrangement providing eflicient excitation of the arrayelements with low quantization sidelobes, comprising:

a plurality of identical groups of radiating elements, each groupconsisting of sixteen radiating elements in five adjacent columns of atriangular grid having four elements in each of the three inner columnsand two elements in each of the outer columns and arranged to besymmetrical about three axes having a common intersection and separatedby sixty degrees;

and means for coupling a different basic excitation signal to each groupof elements, the signal coupled to each group being coupled to all ofthe elements in said group;

whereby the prescribed grouping of elements substantially reduces thenumber of basic excitation signals required, provides efficientexcitation of each element and substantially reduces the quantizationsidelobes.

2. An arrangement of radiating elements in an array antenna as specifiedin claim 1 in Which each radiating element is a circular opening in ametal ground plane terminating a circular waveguide.

3. An array atenna as specified in claim 1 in which the groups ofradiating elements form separate discrete modules which are mechanicallyjoined to form a continuous array of uniformly spaced elements.

References Cited UNITED STATES PATENTS 3,255,457 6/1966 Hannan 343776ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 343-844, 853

